1. Field of the Invention
The present invention relates to a fuel cell system which is adapted for cold start-up, and also relates to a purging method for a fuel cell system.
Priority is claimed on Japanese Patent Application No. 2004-116760, filed Apr. 12, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, fuel cell powered vehicles have been proposed, each of which includes a fuel cell system as a driving power source of the vehicle. As a type of fuel cell system for such vehicles, a fuel cell system is known which includes a predetermined number of stacked cell units, each of which includes an anode electrode, a cathode electrode, and an electrolyte membrane sandwiched therebetween. When hydrogen is supplied to the anode electrode and air (oxygen) is supplied to the cathode electrode, electrical power generation occurs via an electrochemical reaction of hydrogen and oxygen which is accompanied by producing of water. Even though water is mainly formed at the cathode electrode during operation of the fuel cell system, a portion of the water at the cathode electrode may move to the anode through the electrolyte membrane sandwiched between the cathode and the anode.
When the power generation of the fuel cell system is to be stopped, the above-mentioned formed water and humidifying water remain in gas flow paths of the fuel cell units; therefore, when the power generation is stopped while water remains therein, the remaining water may freeze at low temperature, and the frozen water (ice) may block supply and discharge of the reaction gases (hydrogen and air), which leads to degradation of start-up performance at low temperature.
To solve such problem, Japanese Unexamined Patent Application, First Publication No. 2003-203665 proposes a fuel cell system in which a purging operation is carried out in at least one of an anode electrode region and a cathode electrode region when power generation is stopped.
However, in the case in which the fuel cell system is restarted within a short period after stopping of power generation, or in the case in which the temperature of the fuel cell system does not fall significantly because the fuel cell system is used under warm conditions such as in summer, such a purging operation need not be carried out when power generation is stopped. If the purging operation is carried out at every stop of power generation, even under such conditions, endurance of membranes included in the fuel cell may be reduced, and operational life of the fuel cell may be reduced.
Moreover, if the purging operation is carried out at every stop of the power generation, the operation of the fuel cell system is continued for carrying out the purging operation even when the fuel cell powered vehicle stops completely; therefore, the passengers in the vehicle may have unusual sensations, which may degrade the product value of the vehicle.